U.S. patent number 6,452,815 [Application Number 09/865,099] was granted by the patent office on 2002-09-17 for accelerated commutation for passive clamp isolated boost converters.
Invention is credited to Jih-Sheng Lai, Fred C. Lee, Lizhi Zhu.
United States Patent |
6,452,815 |
Zhu , et al. |
September 17, 2002 |
Accelerated commutation for passive clamp isolated boost
converters
Abstract
This invention is an efficient and cost effective bi-directional
DC/DC converter that can effectively reduce the switch voltage
stress (such as a semiconductor) with an accelerated commutation
circuit, and thus allowing a low-cost passive clamp circuit to be
used. Specifically, the invention is a method and system to
accelerate commutation for passive-clamped isolated boost
converters, which can also be a boost mode in a bi-directional
DC/DC converter. A primary circuit has a snubber comprising a
diode, a capacitor and an energy dissipater (such as a resistor or
small buck converter). The primary circuit can be a "full bridge
converter" or a "push-pull converter" or an "L-type converter"
configuration. The commutation of the present invention protects
the primary circuit switches from voltage spikes during switching
conditions. The present invention can shorten a secondary circuit
by turning on at least two switches on the secondary circuit
simultaneously for a minimal calibratable period while primary
circuit diagonal switches turn off. The present invention also has
a means to allow a smooth transition between a choke current and a
primary current. Primary current increases linearly through the
snubber circuit during circuit startup, thus protecting the primary
circuit controllers.
Inventors: |
Zhu; Lizhi (Westland, MI),
Lai; Jih-Sheng (Blacksburg, VA), Lee; Fred C.
(Blacksburg, VA) |
Family
ID: |
26954437 |
Appl.
No.: |
09/865,099 |
Filed: |
May 24, 2001 |
Current U.S.
Class: |
363/17;
363/56.02; 363/56.05 |
Current CPC
Class: |
H02M
3/33584 (20130101); H02M 3/33592 (20130101); H02M
3/337 (20130101); H02M 1/34 (20130101); Y02B
70/10 (20130101); H02M 1/348 (20210501); H02M
1/342 (20210501) |
Current International
Class: |
H02M
3/337 (20060101); H02M 3/335 (20060101); H02M
3/24 (20060101); H02M 003/335 () |
Field of
Search: |
;363/17,24,25,26,39,40,49,56.02,56.05,56.06,56.08,98,127,132,133
;361/91.7 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Han; Jessica
Attorney, Agent or Firm: Naber; John M. Wiltse; Vincent
E.
Parent Case Text
This application is the non-provisional application of provisional
patent application No. 60/270,703 titled, "Accelerated Commutation
for Passive Clamp Isolated Boost Converters," filed Feb. 22, 2001.
Claims
We claim:
1. A system to accelerate commutation for passive-clamped isolated
boost converters comprising: a current source; a primary circuit
comprising at least one pair of switches; a secondary circuit
comprising at least two pairs of diagonal switches; a transformer
connected to the primary circuit and the secondary circuit; and the
primary circuit further comprising a snubber comprising a diode, a
capacitor and an energy dissipater, whereby the primary circuit
switches are protected from voltage spikes during switching
conditions.
2. The system of claim 1 wherein the switches are semiconductor
switches.
3. The system of claim 1 wherein the switches comprise
anti-paralleled diodes.
4. The system of claim 1 wherein the energy dissipater is a
resistor.
5. The system of claim 1 wherein the energy dissipater is a small
buck converter.
6. The system of claim 1 wherein the primary circuit is in a
full-bridge configuration.
7. The system of claim 1 wherein the primary circuit is in a
push-pull configuration.
8. The system of claim 1 wherein the primary circuit is in an
L-type configuration.
9. The system of claim 1 wherein the primary circuit is accelerated
by a system to shorten the secondary circuit of the transformer and
switching voltage spikes are routed into the snubber on the primary
circuit.
10. The system of claim 9 wherein the system to short the secondary
circuit comprises a system to turn on at least two bottom switches
on the secondary circuit simultaneously for a minimal calibratable
period while primary circuit diagonal switches turn off, whereby
the primary circuit current can be transferred to the secondary
circuit quickly and reduce a voltage spike due to primary current
and transformer leakage interaction.
11. The system of claim 1 wherein the system is one
directional.
12. The system of claim 1 wherein the system is bi-directional.
13. The system of claim 1 wherein the snubber further comprises a
means to allow a smooth transition between a choke current and a
primary current.
14. A method for accelerating commutation for passive clamp
isolated boost converter circuit comprised of a transformer
connected to a primary circuit, and a secondary circuit, comprising
the steps of: bypassing at least one pair of diagonal primary
circuit switches during switching; shorting the secondary circuit;
and snubbing the primary circuit using a diode, a capacitor and a
means for dissipating energy, whereby primary circuit switches are
protected from voltage spikes in a switching condition.
15. The method of claim 14 wherein the switches are semiconductor
switches.
16. The method of claim 14 wherein the switches comprise
anti-paralleled diodes.
17. The method of claim 14 wherein the means for dissipating energy
uses a resistor.
18. The method of claim 14 wherein the means for dissipating energy
uses a small buck converter.
19. The method of claim 14 wherein the step of shortening the
secondary circuit comprises the step of turning on at least two
switches on the secondary circuit simultaneously for a minimal
calibratable period.
20. The method of claim 14 wherein the method is one
directional.
21. The method of claim 14 wherein the system is
bi-directional.
22. The method of claim 14 wherein snubbing the primary circuit
further comprises the step of allowing a smooth transition between
a choke current and a primary current.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a DC/DC converter and
specifically to a system and method to accelerate commutation for a
passive clamp isolated boost for a bi-directional DC/DC
converter.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in
automobiles and other vehicles predominately powered by internal
combustion engines (ICEs) is well known. Vehicles powered by
electric motors attempt to address these needs.
Typically, a vehicle propelled by an electric motor can use
batteries or fuel cells to generate the necessary current. Fuel
cells generate electrical power through an electrochemical reaction
of a fuel and oxidant, such as hydrogen and oxygen. Water is the
product of the electrochemical reaction in a fuel cell utilizing
hydrogen and oxygen, a product that is easily disposed. See
generally, U.S. Pat. No. 5,991,670 to Mufford.
The desirability of using electric motors to propel a vehicle is
clear. There is great potential for reducing vehicle fuel
consumption and emissions with no appreciable loss of vehicle
performance or drive-ability. Nevertheless, new ways must be
developed to optimize these potential benefits.
One such area of electric vehicle (EV) development is converting
direct current (DC) generating devices such as fuel cells and high
voltage (HV) batteries to their appropriate load. Ideally, the
current generators (such as HV batteries or fuel cells) and loads
(such as vehicle 12 V powered accessories) would all be at the same
voltage level. Unfortunately, this is not presently the case. For
example, a conventional 12 V voltage system is still needed in an
electric vehicle to power conventional 12 V loads such as lights,
sensors and controllers, while a high voltage bus (for example 300
V) feeds the traction inverter and motor. There is a dual-voltage
power system in the electric vehicle and energy needs to be
transferred bi-directionally between the two voltage systems.
Therefore, a successful implementation of electric traction motor
propelled vehicles requires an effective bi-directional DC/DC
converter. The converter must be bi-directional because the high
voltage bus can be used as a current load during start-up or as a
current generator. Similarly, the 12 V battery can be used as a
current generator or as a load while charging. DC/DC converters are
certainly known in the prior art. Even bi-directional DC/DC
converters are known. See generally, U.S. Pat. No. 5,745,351 to
Taurand and U.S. Pat. No. 3,986,097 to Woods.
In a bi-directional DC/DC converter, one side, the primary side, of
the transformer can be current-fed and the other side, the
secondary side, can be voltage-fed. It is well known in the prior
art that the primary side normally experiences a high voltage
overshoot during a diagonal switch-pair turn-off condition. This
voltage spike needs to be clamped to avoid a voltage overshoot
passing through the switching devices. A passive clamp converter
employs a diode and a capacitor to absorb excessive energy from the
voltage overshoot and a resistor to dissipate the absorbed energy.
Unfortunately, the use of a simple prior art passive-clamped
snubber results in severe limitation in a low voltage (12 V), high
current (e.g., hundreds of amperes) application due to significant
power loss, although it is a simple approach widely used by
industry to resolve the voltage spike issue.
An active clamp in the prior art replaces the resistor in the
passive clamp circuit with a switch to pump back the energy to the
source when the capacitor is not absorbing energy. This recycles
the dissipated energy and improves efficiency, but this technology
is expensive to implement.
In the prior art, bi-directional flyback converters are known to be
best suited for low power applications. Any automobile DC/DC
converter must be able to withstand the extreme environmental
conditions and higher power requirements experienced by many
vehicles. Therefore, there is a desire and a need for an efficient
and cost effective high power bi-directional DC/DC converter.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an
efficient and cost effective high power bi-directional DC/DC
converter that can withstand the vigorous environment of an
automobile.
The invention is a method and system to accelerate commutation for
passive-clamped isolated high power boost converters comprising a
primary circuit having at least one pair of diagonal controllers
(such as switches or diodes); a secondary circuit comprising at
least two controllers (such as switches or diodes); a one
directional or bi-directional transformer connected to the primary
circuit and the secondary circuit; and the primary circuit also
having a snubber comprising a clamping diode, a clamping capacitor
and an energy dissipater (such as a resistor or small buck
converter). The clamping diode can include "push-pull" and "L"
configurations. The commutation of the present invention protects
the primary circuit switches from voltage spikes in a boost
mode.
The present invention shorts the secondary circuit with a method
and system to turn on at least two switches in the secondary
circuit simultaneously for a minimal calibratable period (for
example, 2 microseconds) while primary circuit diagonal switches
turn off, whereby the primary circuit current can be transferred to
the secondary circuit quickly and reduce a voltage spike due to
primary current and transformer leakage interaction.
The present invention also has a means to allow a smooth transition
between a choke (inductor) current and a transformer primary
current. Primary current increases linearly through the snubber
circuit during switching conditions, thus protecting the primary
circuit controllers.
Other objects of the present invention will become more apparent to
persons having ordinary skill in the art to which the present
invention pertains from the following description taken in
conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE FIGURES
The foregoing objects, advantages, and features, as well as other
objects and advantages, will become apparent with reference to the
description and figures below, in which like numerals represent
like elements and in which:
FIG. 1 illustrates a bi-directional full-bridge DC/DC converter
with Accelerated Commutation for Passive Clamping (ACPC);
FIG. 2 illustrates a Timing diagram of the proposed converter;
FIG. 3 illustrates a [t0, t1] Interval;
FIG. 4 illustrates a [t1, t2] Interval;
FIG. 5 illustrates a [t2, t3] Interval;
FIG. 6 illustrates a [t3, t4] Interval;
FIG. 7 illustrates a [t4, t5] Interval;
FIG. 8 illustrates an alternative embodiment using a small buck
converter instead of a clamping resistor (Rc);
FIG. 9 illustrates a Push-pull Converter with Accelerated
Commutation for Passive Clamping (ACPC); and
FIG. 10 illustrates an L-type Converter with Accelerated
Commutation for Passive Clamping (ACPC).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention relates generally to a DC/DC converter and
specifically to a system and method to accelerate commutation for a
passive clamp isolated boost for a high power bi-directional DC/DC
converter. For the present application, high power could be defined
as greater than 1 kW power.
Generally, the operation of inductive storing converters is based
on energy transfer cycles. This includes a period of accumulation
of magnetic energy in an inductive device (such as an inductor or
transformer) through a circuit, followed by a period of restitution
of this energy in a load (such as a typical 12 V load in a car)
through another circuit.
The present invention relates in particular to a DC/DC converter.
This converter can be bi-directional and transforms energy from
primary to secondary circuits and from the secondary to primary
circuits through a transformer. The primary circuit comprises at
least one pair of switches and the secondary circuit has at least
two pairs of diagonal switches. The converter of the present
invention is particularly adapted to function like an electronic
"starter/alternator" for an electric vehicle (EV) although several
other types of applications are possible. The present invention can
boost voltage up from a 12 V battery to a high voltage to start up
a fuel cell powered EV, and then can convert the voltage down from
a high-voltage bus to a low-voltage bus to supply 12 V loads and
charge the battery. The present invention uses a special control
method and system to accelerate transformer current transfer from
one circuit to the other circuit during switching conditions.
In a bi-directional DC/DC converter, one side of the transformer
can be current-fed (for example, having high current to feed the
battery). This is the primary side. The other side, the secondary
side, can be voltage-fed. The energy can be transferred
bi-directionally between the primary side and secondary side.
The present invention is an improvement over the prior art. Due to
the existence of leakage inductance of an isolated transformer in a
current-fed isolated DC/DC converter, the current passing through a
choke, which is an inductor between a DC source and the switches,
generates a high voltage spike across the switching device. This
high voltage spike can damage the circuit during switching
conditions. Specifically, the primary side normally experiences a
high voltage overshoot during a diagonal switch-pair turn-off
condition. This overshoot is simply the multiplication of the
inductance and the rate of the current. To protect the circuitry,
this voltage spike needs to be clamped (or damped) to avoid a high
switch voltage rating for the circuits. The high switch voltage
rating makes the circuitry much more expensive since the circuitry
must be able to withstand the voltage spike.
The control method and system of the present invention can reduce
the clamping energy to the clamp circuit, thus reducing the voltage
spike. This allows the use of less expensive low switch voltage
rating circuitry.
Clamping can usually be characterized in the art as a passive clamp
or an active clamp. Usually, a normal passive clamping circuit,
having a diode, a capacitor and a resistor, results in very low
efficiency. In an alternative active-clamp current-fed isolated
DC/DC converter, the root-mean-square (RMS) current going through
the clamping switch is very Ad high. Active clamping requires more
parallel devices for the active clamp switch and very good
capacitors for the active capacitor to handle such a high RMS
current, adding to the expense of the circuitry.
Therefore, to obtain cost effective clamping circuitry of the
bi-directional DC/DC converter, the present invention has an
accelerated commutation using passive clamping (ACPC). The present
invention provides better efficiency than the normal passive
clamping circuit, but at lower cost than the active clamping
circuit. Although the preferred embodiment of the present invention
is targeted for vehicles propelled by electric traction motors, it
could be used for any type of
DC/DC conversion such as one directional or bi-directional. By way
of example, possible applications can include bi-directional
charging between fuel cell and battery or bi-directional charging
between low voltage battery and high voltage battery.
The present invention is best understood using the variables as
defined below:
Symbol Definition C capacitor V voltage R resistance ACPC
accelerated commutation for passive clamping V.sub.b low voltage
source side V.sub.o high voltage source side V.sub.o to V.sub.b
buck-mode V.sub.b to V.sub.o boost-mode L choke or inductor
arranged on the V.sub.b side S1, S2, S3, S4 switches that act as an
inverter bridge (V.sub.b to V.sub.o in boost-mode) and a rectifier
bridge (V.sub.o to V.sub.b in buck mode) S5, S6, S7, S8 switches
that act as a rectifier bridge (V.sub.b to V.sub.o in boost-mode)
and an inverter bridge (V.sub.o to V.sub.b in buck-mode) Dc-Cc-Rc
passive snubber designed for boost-mode I.sub.L inductor current
L.sub.lk transformer leakage inductance I.sub.P Transformer primary
current Cc snubber clamping capacitor I.sub.S transformer secondary
current R.sub.c snubber resistor D.sub.c clamping diode C.sub.i
input filter capacitor N negative node P positive node A node A B
node B C node C D node D T main transformer n.sub.T number of turns
C.sub.o output filter capacitor T.sub.s high frequency switching
period
The method and system of the present invention can accelerate the
circuit by shorting the secondary side of the transformer and route
switch voltage spikes to a snubber on the primary side. Generally,
the method and system of the present invention is to turn on two
bottom switches on the secondary circuit (see Switches S636 and
S738 in FIG. 1) simultaneously for a minimal calibratable period
(for example, 2 micro-seconds) when the primary diagonal switches
turn off. Thus, at least one pair of diagonal primary circuit
switches are bypassed during switching. conditions. This switching
condition transfers primary current to secondary current very
quickly since the whole voltage just applies to the leakage
inductance of the transformer. Further, the voltage spike due to
primary current and transformer leakage interaction can be largely
reduced.
To illustrate the preferred embodiment of the invention, FIG. 1
shows a bi-directional full-bridge DC/DC converter with accelerated
commutation for passive clamping (ACPC). As shown in FIG. 1,
full-bridge in the primary side has Switches S126, S228, S330, and
S432. The switches in the overall circuit can be semiconductors and
may also have anti-paralleled diodes that are well known in the
prior art. In the secondary side, the full-bridge has Switches
S534, S636, S738, and S840. A V.sub.b 20 represents a low voltage
current source such as a battery, while a V.sub.o 22 represents a
high voltage current source such as a generator. It should be noted
that in all the circuit figures for the present invention the
arrows represent the flow of current based on the status of the
switches or controllers for that time interval. The dashed lines
(124) in each figure represent the branch is switched off in that
interval and does not carry any current. The present invention not
only provides bi-directional DC/DC power flow control between the
V.sub.b 20 and V.sub.o 22, but also isolates the V.sub.b 20 and
V.sub.o 22 sources by a transformer (T) 64. A choke (L.sub.f) 24 is
positioned on the low voltage side (V.sub.b 20 side). When power is
delivered from V.sub.b 20 to V.sub.o 22, it is called "boost-mode."
Switches S126, S228, S330,and S432 act as an inverter bridge, and
Switches S534, S636,S738, and S840 act as a rectifier bridge. The
inverter bridge inverts a DC/DC voltage to an AC voltage, and the
rectifier acts in reverse. Although switches are illustrated for
the preferred embodiment, diodes or other types of controllers
known in the prior art could also be used. Power delivered from
V.sub.o 22 to V.sub.b 20 is called "buck-mode." In "buck-mode,"
Switches S534, S636, S738, and S840 act as the inverter bridge, and
switches S126, S228, S330, and S432 act as the rectifier bridge.
The primary circuit also has a clamping diode (Dc) 42, a snubber
(clamping) capacitor (Cc) 44 , and an energy dissipater such as a
resistor (Rc) 46. The Dc 42-Cc 44-Rc 46 combine to form a passive
snubber (also known as a damper) and designed to be most effective
in the boost-mode. The effect of the snubber can be referred to as
"snubbing." When the voltage spike occurs during the inverter
bridge transition, the passive circuit absorbs the energy and
clamps the voltage to a lower value.
Additionally, the circuit has an Input Capacitor (C.sub.i) 48, an
output capacitor (C.sub.o)50, a Node A (A) 52, a Node B (B) 54, a
Node C (C) 56, a Node D (D) 58, a choke current through inductor
(I.sub.L)60, primary current (I.sub.p)62, the transformer (T) 64, a
Transformer Secondary Current (I.sub.s)66, Transformer Leakage
Inductance (L.sub.IK)68, and the Ratio of Transformer Wire Turns
1:n.sub.t 70.
FIG. 2 illustrates the timing waveform diagram for the circuit
illustrated in FIG. 1 during time interval T.sub.s 72. As shown in
FIG. 2, the status of various switches during the T.sub.s 72 is
represented by "S1, S2" 74, "S3, S4" 76, and "S6, S7" 78. When the
switch is "on," a line is drawn above a base line. "I.sub.S1,
I.sub.S2 " 80 shows the current during the T.sub.s 72 for switches
S126 and S228 respectively. "I.sub.S3, I.sub.S4 " 82 shows the
current during the T.sub.s 72 for switches S330 and S432
respectively. An I.sub.L 84 is the current through the inductor for
the corresponding interval. An I.sub.p 86 is the current through
the primary side of the T 64, I.sub.Cc 88 is the current through
the snubber circuit (Dc 42-Cc 44-Rc 46). A U.sub.pn 92 represents
the voltage between a Positive Node (P) 94 and a Negative Node (N)
96 respectively. The timing waveform diagram is best understood
dividing time interval T.sub.s 72 into subintervals to 98, t.sub.1
100, t.sub.2 102, t.sub.3 104, t.sub.4 106, t.sub.5 108, t.sub.6
110, t.sub.7 112, t.sub.8 114, and t.sub.9 116.
During the [t.sub.0 98, t.sub.1 100] interval illustrated in FIG.
3, Switches S126, S228, S330,and S432, are turned on. Switches
S534, S636, S738,and S840, are turned off. L.sub.f 24 is charged by
V.sub.b 20 and the I.sub.L 60 increases linearly.
During the [t.sub.1 100, t.sub.2 102] interval illustrated in FIG.
4, Switches S330 and S432 are turned off to illustrate a diagonal
switch shutoff. It is during this switching condition time interval
that the voltage spike is normally experienced. Also during the
[t.sub.1 100, t.sub.2 102] interval, S636 and S738 are turned on,
shorting the secondary side of the transformer T 64. Due to the
existence of a L.sub.lk 68, the I.sub.p 62 cannot instantly change
to I.sub.L 60.Therefore, I.sub.p 62 increases linearly to I.sub.L
60 while the current through the snubber circuit (Dc 42-Cc 44-Rc
46) I.sub.cc 88 linearly decreases to zero. During the [t.sub.1
100, t.sub.2 102] interval, the difference in current between
I.sub.L 60 and I.sub.p 62 goes into C.sub.c 44, avoiding the high
voltage spike across the P node 94 and N node 96 bus. Using the
design of the present invention, the Dc 42-Cc 44-Rc 46 snubber
circuit protects Switches Si 26, S228, S330, and S432 during this
interval. It does this properly not only by turning off switches on
the primary side, but also by providing a means for a smooth
transition between I.sub.L 60 to I.sub.p 62 via the snubber circuit
(switching condition). The switches are turned off for the minimal
calibratable time in this embodiment and could, by way of example,
be 2 microseconds.
Next, at time interval [t.sub.2 102, t.sub.3 104] illustrated in
FIG. 5, Switches S636 and S738 remain "on," thus the secondary side
of the T 64 remains shorted. During this interval, I.sub.P 62
equals I.sub.L 60 and U.sub.PN 92 is inverted.
Next, time interval [t.sub.3 104, t.sub.4 106] illustrated in FIG.
6 is an energy transferring interval. Here, switches S738 in and
S840 are turned off at time t.sub.3 104.Then I.sub.S 66 goes
through body diode Switches S534 and S636, delivering energy from
the primary side to the secondary side, (i.e., "boost-mode").
At time t.sub.4 106, during time interval [t.sub.4 106, t.sub.5
108] illustrated in FIG. 7, switches S330 and S432 are switched
"on." The circuit of the primary side of the T 64 is shorted again
to store energy to the L.sub.f 24. Consequently, I.sub.L increases
linearly. Due to the reflected voltage V.sub.o 22 applies on
L.sub.lk 68, the I.sub.p 62 is reset to zero.
As shown in FIG. 1, partial energy in C.sub.c 44 is dissipated on
the R.sub.c 46.This causes some loss of energy and thus can lower
overall circuit efficiency. As an alternate embodiment of the
present invention, FIG. 8 illustrates a small "buck" converter 118
replacing the R.sub.c 46 to recycle the power in the snubber
circuit back to the source if efficiency is a concern. The small
"buck" converter" having a Lcb 126 (inductor filter), Scb 128
(active switch), and Dcb 130 (rectifier diode).
Other alternate embodiments using different types of converters are
also possible. FIG. 9 illustrates a variation using a "push-pull"
converter with Accelerated Commutation for Passive Clamping (ACPC).
Here the choke diode D.sub.c 42 is replaced with a "push-pull"
configuration known in the prior art using D.sub.c1 120 and
D.sub.c2 122. FIG. 10 illustrates yet another embodiment using an
L-type Converter with Accelerated Commutation for Passive Clamping
(ACPC). Here, the choke diode D.sub.c 42 is replaced with a
"L-type" configuration known in the prior art using D.sub.c1 120
and D.sub.c2 122. The push-pull and L-type configurations, which
have a different number of inductors and switches in the primary
side, offer more choices for different power and voltage
applications.
The above-described embodiments of the invention are provided
purely for purposes of example. Many other variations,
modifications, and applications of the invention may be made.
* * * * *